On Tue, 2009-04-28 at 14:21 +0200, Gilles Chanteperdrix wrote:
> Philippe Gerum wrote:
> > On Tue, 2009-04-28 at 07:50 +0200, Gilles Chanteperdrix wrote:
> >> Hi,
> >>
> >> currently, the situation is this:
> >> - the timing core uses an approximate value of the cpu frequency (using
> >> xnarch_llmulshft) to do conversions between tsc and ns;
> >> - the APIC timer reprogrammation still uses imuldiv, that is a more
> >> exact cpu frequency, coupled with an approximate APIC frequency
> >> (obtained from ipipe_request_tickdev), to do the conversion between tsc
> >> delays and APIC delays;
> >> - the posix skin uses llimd to do conversions between timespec and
> >> timeval structures and tsc values
> >> - the posix and native skins in user-space still use llimd to do
> >> conversions between ns and tsc (only if using tsc in user-space).
> >>
> >> These are causes of unprecisions, I think we should fix this, though it
> >> is unlikely to be the cause of the high jitters observed by Vikesh with
> >> 2.5-rc1.
> >>
> >
> > Ack, we definitely should. Working on allowing userland to use
> > llmulshift for timing computations for the native skin here, as a first
> > step. For that purpose, xnsysinfo_t has just been extended to carry the
> > pre-scaled values needed for llmulshft. I just don't want to have to
> > issue a syscall for those conversions to take place.
>
> Actually, the pre-scaled values could be re-computed in user-space
> without changing the ABI.

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We don't care that much about changing the ABI between -rc1 and -rc2
actually, so it mostly depends whether we want to backport the fix to
2.4 regarding the tsc->ns conversion, since we do not have any nodiv
implementation for the ns->tsc one there yet.
--
Philippe.
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